how to report missing files in a list, usable in python script?
You could try a variation of the following code:

import bpy
import os

def list_missing_blend_libs():
    return [bpy.path.abspath(lib.filepath) for lib in bpy.data.libraries
            if not os.path.exists(bpy.path.abspath(lib.filepath))]


Using bpy.path.abspath, it only lists absolute path of missing libraries in bpy.data.libraries.